With the advent of digital communications technology, many TV program streams are transmitted in digital formats. For example, Digital Satellite System (DSS), Digital Broadcast Services (DBS), and Advanced Television Standards Committee (ATSC) program streams are digitally formatted pursuant to the well known Moving Pictures Experts Group 2 (MPEG-2) standard. The MPEG-2 standard specifies, among others, the methodologies for video and audio data compressions which allow multiple programs, with different video and audio feeds, multiplexed in a transport stream traversing a single transmission channel. A digital TV receiver may be used to decode an MPEG-2 encoded transport stream, and extract the desired program therefrom.
MPEG-2 Background
In accordance with the MPEG-2 standard, video data may be compressed based on a sequence of groups of pictures (GOPs), made up of three types of picture frames—intra-coded picture frames (“I-frames”), forward predictive frames (“P-frames”) and bilinear frames (“B-frames”). Each GOP may, for example, begin with an I-frame which is obtained by spatially compressing a complete picture using discrete cosine transform (DCT). As a result, if an error or a channel switch occurs, it is possible to resume correct decoding at the next I-frame.
The GOP may represent additional frames by providing a much smaller block of digital data that indicates how small portions of the I-frame, referred to as macroblocks, move over time.
An I-frame is typically followed by multiple P- and B-frames in a GOP. Thus, for example, a P-frame occurs more frequently than an I-frame by a ratio of about 3 to 1. A P-frame is forward predictive and is encoded from the I- or P-frame that precedes it. A P-frame contains the difference between a current frame and the previous I- or P-frame.
A B-frame compares both the preceding and subsequent I- or P-frame data. The B-frame contains the average of matching macroblocks or motion vectors. Because a B-frame is encoded based upon both preceding and subsequent frame data, it effectively stores motion information.
Thus, MPEG-2 achieves its compression by assuming that only small portions of an image change over time, making the representation of these additional frames extremely compact. Although GOPs have no relationship between themselves, the frames within a GOP have a specific relationship which builds off the initial I-frame.
The compressed video and audio data are carried by continuous elementary streams, respectively, which are broken into access units or packets, resulting in packetized elementary streams (PESs). These packets are identified by headers that contain time stamps for synchronizing, and are used to form MPEG-2 transport streams. For digital broadcasting, multiple programs and their associated PESs are multiplexed into a single transport stream. A transport stream has PES packets further subdivided into short fixed-size data packets, in which multiple programs encoded with different clocks can be carried. A transport stream not only comprises a multiplex of audio and video PESs, but also other data such as MPEG-2 program specific information (sometimes referred to as metadata) describing the transport stream. The MPEG-2 metadata may include a program associated table (PAT) that lists every program in the transport stream. Each entry in the PAT points to an individual program map table (PMT) that lists the elementary streams making up each program. Some programs are open, but some programs may be subject to conditional access (encryption) and this information is also carried in the MPEG-2 transport stream, possibly as metadata.
The aforementioned fixed-size data packets in a transport stream each carry a packet identifier (PID) code. Packets in the same elementary streams all have the same PID, so that a decoder can select the elementary stream(s) it needs and reject the remainder. Packet-continuity counters may be implemented to ensure that every packet that is needed to decode a stream is received.
Use of personal video recorders (PVRs), also known as digital video recorder (DVRs), such as TiVo and ReplayTV devices, provide conveniences to viewers of television programming. For example, if a PVR user is watching an in-progress broadcast program, the user may rewind the recorded portion of the program to a certain point that has already been broadcast or may pause for subsequent playing, rewinding and/or fast-forwarding. Similarly, for programming that has already been broadcast and recorded, the user may also apply any of these PVR functions.
The cable TV industry has been fervently pursuing a “network PVR (NPVR)” approach allowing the user to perform the analogous PVR functions through use of a network, rather than a local PVR device. In fact, a network architecture and functionalities for implementing such an NPVR approach have been developed and are described, e.g., in copending commonly assigned application Ser. No. 10/263,015, filed on Oct. 2, 2002, hereby incorporated by reference.
In some instances, however, the aforementioned PVR functions may have an adverse effect on advertisers. For instance, if a PVR user decides to rewind or fast-forward past a commercial, the user would hardly realize the impression of the commercial at a high rewind or fast-forward speed, and thus appreciate its effect intended by the advertiser.